1. Field of the Invention
The present invention relates to watthour meter systems for measuring the energy consumed by loads connected to the lines of power systems, and more particularly pertains to a new method and system for improving the operational safety, reliability, and functionality of electrical power consumption monitoring devices, especially during a changeover of the monitoring devices.
2. Description of the Prior Art
In general, the use of watthour meter systems is known in the prior art. U.S. Pat. No. 5,834,932, issued on Nov. 10, 1998 to Gregory R. May, contains an excellent description of the types and forms of watthour meter systems known in the prior art. The disclosure of the U.S. Pat. No. 5,834,932 patent is hereby incorporated by reference in its entirety into this disclosure to the extent that it is not inconsistent with the present invention.
Electrical energy consumption measuring systems may take the form of an electrical watthour meter system. A typical electrical watthour meter may include both voltage sensing windings and current sensing windings internal thereto. The voltage sensing windings are each constructed and arranged so that the electrical meter may measure a first voltage appearing across that voltage sensing winding and determine a second voltage appearing across at least two lines of an electrical circuit system. Each current sensing winding of the electrical meter is constructed and arranged so that the electrical meter may measure a first current driven through that current sensing winding and determine a second current driven through a line of the electrical circuit system.
An electrical watthour meter is typically seated into (and removed from) a meter socket, and the mounting and removal from the socket may occur even while the power line to which the meter socket is connected remains “live” or “hot” with power. The meter socket is typically configured to matingly receive elements of the electrical meter. The meter socket may include a plurality of socket terminals, which may take the form of jaw-like structures sometimes referred to as “jaws”. The electrical meter may include a plurality of conductor lugs, which may take the form of blade-like structure sometimes referred to as “stabs”. The jaws of the meter socket removably receive the stabs of the electrical meter when the meter is mounted on the meter socket, provided that the conductor stabs are in registry with the socket jaws.
For several decades, it was common practice to utilize personnel to periodically visit each meter and physically read the meter for the purposes of billing. More recently, systems have been developed to not only automate this process, but also to provide load management capabilities. One such system is a data transmitter which employs Power Line Carrier (PLC) communication techniques to facilitate communication between the electrical meter and an electrical substation. The data transmitted from the electrical meter to the substation can then be translated for retransmission to a central office using a variety of communication channels including telephone land lines, cellular telephones, and satellite systems. These systems can be either external or internal to the electrical meter. As an illustrative example, Hunt Technologies of 6436 County Road 11, Pequot Lakes, Minn. 56477, sells data transmission systems of this type under the trade name TURTLE, although other systems are available and may be employed.
Because of the significant advantages provided by this type of automated system, many utility companies are electing to upgrade many if not all of their electrical meters to reduce the labor required to read the meters, as well as to gain information about power consumption for each load or customer, and, in the instances of two-way communication systems, obtain some load management capabilities as well. Physically changing out these meters is not only labor intensive, it can also be extremely dangerous when the power line to which the meter socket is connected is live or hot with power. While the electrical meter can be removed by simply pulling the meter away from the meter socket and replaced by pushing a meter onto the meter socket regardless of whether or not power is applied or whether a load is connected to the output of the meter, the presence of live power presents a hazard that cannot always be avoided. And the danger associated with the change out of the electrical meter significantly increases in the case of higher voltage poly phase power lines, such as the 480 volt three phase systems commonly used for agricultural and industrial services. While some utilities require that an entire electrical service be disconnected from live power prior to a lineman changing out an electrical meter, the disconnection may not be practical for a variety of reasons, such as when the electrical service provides power for an entire shopping center or industrial installation.
Installing and maintaining new or retrofit electrical meters on live power services exposes the system to at least two separate sources of potentially catastrophic failures. These failures include failure of a metal oxide varistor (MOV) in the meter, and arcing between the conductors of the service during installation of the meter, both of which can lead to phase-to-phase or phase-to-neutral faults and potentially cause a full flashover event.
MOVs are fast acting variable resistors which present a relatively higher impedance at normal operating voltages, and a relatively lower impedance at higher than normal operating voltages. As will be readily appreciated by those skilled in the art, MOVs are commonly used to protect circuits from over-voltage conditions. Illustrative examples of such over-voltage conditions are a lightning strike, ungrounded wye/delta phenomena, and ferro-resonnance. As useful as MOVs may be in protecting circuits, they do have at least one potential shortcoming, specifically a negative temperature coefficient. Thus as a MOV becomes hotter, its resistance decreases and the MOV will carry more current, which will in turn increase the heat generated by the MOV. This can lead to a thermal runaway in which the MOV may rupture from the excess heat and may expel hot conductive materials and gases. Because of the relatively close physical proximity of the electrical terminals, in both the meter and the junction box on which the meter socket is typically mounted, the conductive gases may reduce the insulating nature of the air in the meter or junction box and induce a flashover. For the purposes of this disclosure, a flashover is defined as a disruptive discharge around or over the surface of an insulator. As can be appreciated, a flashover at relative high voltages and currents can be extremely hazardous and destructive.
A second failure mechanism that is sometimes present in removing, maintaining, or installing electrical meters occurs when the electrical service to the meter is live, and a load is present on the output of the meter. As a terminal of the meter either makes or begins to break contact with a terminal of the meter socket for at least one phase of the electrical power, arcing can occur. The arcing ionizes the surrounding atmosphere making the air more conductive, which then promotes increased arcing, flashover, and even the explosive destruction of the meter and junction box.
As discussed above, attempts to solve these problems have centered on disconnecting the power supplied to the junction boxes of the meter socket or disconnecting the entire service, including the power transformers, before changing out meters. However, this practice inevitably disrupts the supply of power to at least one, and in some occasions many, customers, and for commercial customers this may be unacceptable during normal business hours. In addition, energizing the high voltage side of the power transformers one phase at a time can sometimes create significantly higher than normal voltages on the secondary, contributing to the failure of the meter,